The present invention relates to a speech recognition method for a speech recognition apparatus such as a speech typewriter, and particularly to a speech recognition apparatus which is suitable for discriminating similar types of speech, is able to readily investigate the cause of incorrect recognition, and is suitable for systematically improving its performance.
As described in IEEE, Trans. on Acoustic Speech and Signal Processing ASSP-23, No. 1 (1975) pp 67-72, conventional speech recognition apparatuses use a method in which standard patterns have been previously prepared for all the categories of a certain type of speech to be recognized, and an input pattern is matched with each of these standard patterns so as to obtain the result of recognition with the highest similarity. In addition, as a measure that can be taken when sufficient similarity cannot be obtained by the matching undertaken in the above-described method, a method is employed as described in Japanese Patent Laid-Open No. 36824/1980 in which patterns carrying partial features of a type of speech are stored as partial standard patterns, and a particular portion of an input pattern is matched with the abovedescribed partial standard patterns so as to obtain a final result of recognition on the basis of the results of the comparison.
In the above-described prior art, information used as a standard of recognition is held in the form of standard patterns. Recognition is performed on the basis of the matching with the standard patterns, but such a method has a problem that speech categories naturally having similar acoustic features cannot be discriminated well. In addition, since the features of a particular type of speech are implicitly involved in the standard patterns, a person cannot judge whether the process undertaken in the course of recognition is correct or not. In other words, when incorrect recognition occurs, it can only be judged that the main cause is the poorness of the standard patterns There are therefore problems that the performance of a recognition apparatus has to be improved by a method of trial and error and, thus, no knowledge is accumulated and no improvements in performance can be expected, as well as with regard to the fact that the internal condition of a recognition system cannot be displayed on a monitor and hence its performance cannot be improved systematically.